Concentration of electrolyte from dilute washings by electrodialysis in a closed system

ABSTRACT

Work from electroplating or metal finishing is washed by immersion in a series of solutions. The dragout of electrolyte which remains in the wash solutions is balanced by the transfer of electrolyte from each wash solution to a previous solution of the sequence or to a solution of the original bath concentration. This transfer of electrolyte is by electrodialysis in which a circulating stream of each wash solution acts as a diluting stream in relation to a circulating stream of a previous solution and as a concentrating stream in relation to a subsequent solution. Extremely high ratios of concentration are achieved between wash stages by operation of the electrodialysis stack under conditions of membrane polarization.

United States Patent 1151 3,674,669

Tuwiner 1 July 4, 1972 [541 CONCENTRATION OF ELECTROLYTE 3,481,85112/1969 Lancy ..204/1s0 P FROM DILUTE WASHINGS BY OTHER PUBUCATIONSELECTRODIALYSIS IN A CLOSED SYSTEM Wilson, Deminerlization byElectrodialysis, pp, 12- 18, TD433p7c12, 1960 [72] Inventor: Sidney B.Tuwiner, Baldwin, NY. [73] Assignee' RAl Research Corporation Lon lslandPrimary Examiner-John Mack Cit N Y g Assistant Examiner-A. C. PrescottAttorney-Kenyon & Kenyon Reilly Carr & Chapin [22] Filed: April 1, 197057 ABSTRACT [21] App]. No.: 24,602 1 Work from electroplating or metalfinishing is washed by immersion in a series of solutions. The dragoutof electrolyte [52] "304/180 204/301 which remains in the wash solutionsis balanced by the transfer [51] 'f Cl "Bold 13/02 of electrolyte fromeach wash solution to a previous solution Fleld 0 Search P, of theequence o to a solution of he original concentra tion. This transfer ofelectrolyte is by electrodialysis in which a References Clledcirculating stream of each wash solution acts as a diluting stream inrelation to a circulating stream of a previous solution UNITED STATESPATENTS and as a concentratin stream in relation to a subsequent solu- 22,802,344 8/1957 Witherell ..204/180 P X 2,848,403 8/1958 Rosenberg..204/l 80 P Extremely high ratios of concentration are achieved between218601095 1 1/ Kan et a] ""204/180 P wash stages by operation of theelectrodialysis stack under 2,863,813 12/1958 Juda et al. ....204/180 Pconditions of membrane polarization 3,124,520 3/1964 .luda ....204/180 P3,459,650 8/ 1969 Hiraiwa et a1. ..204/180 P 13 Claims, 1 Drawing Figurel 9 /0 J I11 '1 F 1 I 1 1 l [3 1 l l Mqtsx/ I My, 775m? 779/: Paw/ HJ/VpJ L... l

2 41/ 6? /ILVZE/Q k k a CONCENTRATION OF ELEC'I ROLYTE FROMDILU'IEWASHINGS BY ELEGTRODIALYSIS INA CLOSED SYSTEM- BACKGROUND OF THEINVENTION In the electroplating and metal finishing industry, metal ormetallized plastic which leaves the plating or chemical bath carriessome of the solution by surface adhesion. This is called dragout anditmust be removed by washingwith water or with an aqueous solution. Ifthecomponents of the finishing bath are not removed, the work may bestained or corroded and, in some caseswhere the components are toxic,they may be hazardous to health, particularlyin thecase of cyanides andchromates.

The washing may be'bya flowing stream of water whenthe water isplentiful and inexpensive and'where the disposal of large volumes ofdilute waste solution isnot a problem. Increasingly; however; therearelimitations to the availability of water of suitable quality and to. thevolume of waste solution.- This is true particularly when lthe wastemust betreated' in accordance with requirements of' federal, state andmunicipal authorities and where thecost of'treatment is-directly relatedto the volume of the waste:

It iscustomary toachievea saving in thevolumeof'waste solution fortreatmentandin thevolume-of water forwashingi by utilizinga sequenceotlwashing' steps in eachof'which' the work is immersed in thewash"solution and' then removed, allowing the solution to drain foraprescribed time. Insuch a wash system each of the washsolutions ismoredilute than the previousoneso that the concentration of the solutediminishes in the solution dragout.

The advantage of such a system over the simpler system employingatlowin'g. stream is that most of the dissolved material is removed inthe earlier"stages-:at'arelatively high level or concentration. Usuallyit is-a practice to add freshwater to the final rinse solution tank-zAs-the volume-of this solution increases there is an overflowinto thenext more concentrated solution tank which, in turn, overflows to thenext until from the first wash solution tank theoverflow is sent toa'waste treatment system and/orto the sewer.

For any given numberof washingstages thereis a washingefiiciency whichisdefined as the percentage ofdissolved material in the work going to' thefii'sfwash tank which is removed from the work by thesystemuThegreaterthe-amount of wash water and waste the greater istheefi'iciency. The:

washing efficiency isgreater, orthe'volume-of waste water is less, thegreater the number of washing stages. However, beyond three'or fourstages;the*gain is marginal in'relation to the increase in capital andoperating costs.

For the treatment of the 'wasteeffluent solutions-there are methods ofchemical andbiological conversion to eliminate noxious components ofthe: stream or to render'them innocuous. Adsorption by activatedcarbonand by ion exchange resins may accomplishthe removaIQ-Economicconsiderations usually necessitate the regeneration of the-adsorbent,always at substantial cost. Electrodialysis is also a 'method which'hasbeen proposed. In this methodithe electrolyte components of the wastestream are removed and recovered in'a more concentrated stream for reuseor disposal.

To illustrate the methods of the priorart for the treatment of platingand metalfinishingwastes, we -may select as typical, the waste from theplating of copper with cyanide, nickel plating solution waste andthewaste solutions from the chrome r pickling of brass mill products;

For the treatment of copper cyanidewastes the usual prac tise is tochlorinate by treatment 'with'calcium-hypochlorite.

Cyanides are readily oxidiiedto cyanates which; though far less toxicthan the cyanides, areusually oxidized further to produce nitrogen'gas'and carbon'dioxide-or carbonates.=Ox'-' idation of the cyanates ispossible with adequate'contact'time and pH control. Inthe combinedtreatment the copper andcyanide values are lost and thecostof reagentchemicals is considerable. The wasterafter-ztreattnent, must-be heldlongenough to be tested to determine that it is innocuous within thecognizant'specifications'.

Nickel plating solution waste is treated with a lime slurry to apredetermined pH to precipitate calcium sulfate and basic salts ofnickel. The'solids are removed in a clarifier from which the clearoverflow is suitable for reuse while the underflow may be collected inlagoons or it may be removed after dewatering. Very little of the nickelvalues are recovered.

The dilute waste from the'chrome pickling of brass products containshexavalent chromium, trivalent chromium, copper and'zinc sulfates andsulfuric acid. This is treated first with sulfur dioxide or sodiumsulfite in an amount which is required stoichiometrically to reduce thehexavalent chromium to trivalent chromium sulfate. Lime is then added toraise the pH to 8-9 to precipitate basic salts of chromium, copper andzinc and calcium sulfate. The water is recovered in the overflow of aclarifier, the underflow going to a lagoon or dewatering filter.

It is obviously advantageous to reconcentrate the components of wastewash solutions without changing them chemically. Electrodialysis is amethod which is known to accomplish'this concentration by the transferof the component ions through ion-selective membranes under the drivingforces resulting from an electric field.

An electrodialyzer consists of a stack of membranes separated by plasticspacers which enclose cells through which the streams are circulated.The membranes are of two kinds: anionselective and cation-selective,which alternate in the stack. The electric field is induced byelectrodes, one at each end ofthe stack,- ananode and a cathode.

The force on the cations isin the direction of the field while that onthe anions'isin the opposite direction. Those cells which are bounded bymembranes such that the field is in the direction from anion-selectiveto cation-selective are such that the electrolyte isdepleted. Wherethefield is in the direction from cation-selective to anion-selectivethe solution tends to become more concentrated.

The two types of cells alternate in order in the stack and the solutionmanifold openings are designed to feed each of the two streams to theappropriate group of cells. One is the concentrating stream and theother is the diluting stream. Usually the two end cells which enclosetheelectrodes are furnished with electrode rinse streams which are separatefrom the two principal process streams. These rinse streams are usuallyrecycled.

By these means there is achieved a transfer of electrolyte from thediluting stream to the concentrating stream. Ordinarily the removal ofelectrolyte, a single electrodialysis stack unit, is limited to from 40to 50 per cent of that which enters the stack. It is obvious, therefore,that to achieve the very low concentrations which are required in thecase of certain'waste components it is necessary, in the prior art, toemploy numerous stages.

To illustrate: if an effluent stream contains ppm of cyanide, and if itis desired to discharge a stream to waste containing no more than 1 ppmwith an electrodialysis system which achieves a 50 per cent reduction ineach electrodialysis stack unit it is necessary to employ seven of suchunits in series. Each of these units requires its own individualpumping'system and electrodes.

Many of the plating shops and metal finishing establishments are smallin size'and hence cannot afford the complexity of sucha system and itsattendant cost. As a consequence, electrodialysis systems are not usedextensively in waste treatment in this industry.

Furthennore, the ratio of demineralization in the methods of the priorart is limited by the membrane transference nunibers'. To illustrate: ifthe transference number 'of the cations in the'cation selectivemembranes and that of the anions in the anion-selective membranes areboth unity there is no limit to the ratio of concentration which can beachieved so long as there is provided an'amount of membrane area andelectrical energy which is adequate in relation to the volume ofsolution and its concentration.

On the other hand, if the transference numbers are less than unity aportion of the current through the cation-selective membrane is carriedby anions moving from the concentrated, to the dilute, solution and aportion of the current through the anion-selective membrane is carriedby cations moving also from the concentrated, to the dilute, solution.This reverse flow of the ions through each membrane persists even whenthere is zero current flow and it is then generally ascribed todialysis, or diffusional transfer of electrolyte from a higher, to alower, concentration.

Inasmuch as the deficiencies of transference of the counterions in thetwo types of membranes are additive they impose a limit to the ratio ofconcentration which can be achieved with the methods of the prior art.Specifically, the transference numbers of electrodialysis membranes fallfrom unity increasingly as the electrolyte concentration is increasedand as the ratio of concentration is increased. The ratio ofconcentration is therefore, self-limiting.

The deviation of the transference numbers of the membranes from unity ismost severe in those systems which include polyvalent ions andespecially when these ions are very strongly held by the membranes. Whenthese polyvalent ions are adsorbed by the membrane they not onlyneutralize the original electric charge of the fixed ionic sites of themembrane polymer but they contribute an excess of the opposite charge.This tends to convert an anion-selective membrane to one which iscation-selective and vice versa.

For example, in the electrodialysis of sodium cuprocyanide solutions,the complex cyanide ions, such as Cu(CN) are adsorbed by theanion-selective membranes. These ions are strongly held and tend toneutralize the positively charged quarternary ammonium ions of thepolymer and to produce a negative charge which tends to exclude mobileanions and promote the counter movement of sodium ions which carry mostof the electric current in the membrane. Although it has been possibleto concentrate cuprocyanide solutions by electrodialysis as in US. Pat.No. 3,357,823, the current efficiencies are extremely poor when theconcentrations are high.

Similar considerations are applicable to the electrodialysis of chromiumwaste solutions in which the trivalent chromium ion is adsorbed by thecation-selective resin with consequent diminution of the cationtransference number and also to the electrodialysis of solutions whichcontain complex fluorides of iron or aluminum or the polymericphosphoric acid salts.

It is therefore an object of this invention to provide an improvedmethod and apparatus for washing objects having adherred electrolyte andtreating the wash solutions.

A particular object is to maintain the effectiveness of wash solutionsby the efficient use of electrodialyzers.

These and other objects of this invention will be apparent from thefollowing detailed description and accompanying drawing.

THE INVENTION This invention is directed to a method and apparatus forwashing adherred electrolyte from a workpiece, in which the efficiencyof the wash solutions is maintained by continuously recirculating thewash solutions through a series of electrodialyzers. The workpiece isremoved from a treating bath and rinsed in a series of wash baths. Thesolution from the first wash bath is passed through a first dialyzerwhich reduces the electrolyte content of the solution by transferringions to a circulating stream from the treating bath. The solution fromthe first wash bath is then passed through a second dialyzer whichincreases the electrolyte content of the solution by taking up ions froma circulating stream from the second wash bath. Extremely high ratios ofconcentration are achieved between the wash stages by operation of theelectrodialyzers under conditions of membrane polarization.

The present invention is further illustrated by reference to the FIGUREwhich represents schematically a system for the washing of work from aplating or metal finishing operation. This work is removed from tank 1which contains the plating or finishing solution. It then is immersedsuccessively in wash tanks 2 and 3 with appropriate drainage afterleaving each tank. Streams 9, 10 and 14 represent solution which isdragged with the work at each stage. The volume of solution in thesethree streams is approximately the same but the concentration is lowerin each successive stage. Additional stages beyond those illustrated maybe used in which each additional stage between tank 2 and tank 3 isconstructed and operated in the manner set forth below for tank 2. Tank1 may also represent a tank from which electrolyte is eventuallyrecovered in accordance with known methods.

Pumps 4, 5 and 6 circulate streams 1 l, 12 and 13 out of, and back tothe tanks 1, 2 and 3. Stream 11 is the concentrating stream passedthrough dialyzer stack 7. Stream 12 is the diluting stream passedthrough dialyzer stack 7 and also the concentrating stream passedthrough electrodialyzer stack 8. Stream 13 is the diluting stream passedthrough stack 8. Alternatively stream 12 may be passed through dialyzer7 and then back into tank 2, and a separate stream from tank 2 may bepassed through dialyzer 8 and then back to tank 2. However, forefficiency of operation the embodiment of the Figure as illustrated ispreferred.

It is thus preferable, though not essential, to mount theelectrodialyzer stacks 7 and 8 on a single frame and to provide manifoldconnections for the flow of stream 12 from the one to the other. StreamI2 flows in parallel relation with stream 11 through dialyzer 7 andsimilarly streams 12 and 13 flow in parallel relation through dialyzer8. The design of the electrodialyzer separators, electrodes, endcompression heads and manifold connections are those which are wellknown and the membranes are those which are used in commercialelectrolysis in the prior art. Note, for example, the construction andoperation of such units as set forth in Diffusion and MembraneTechnology, Reinhold Publishing Company, 1962.

Inasmuch as the three streams 11, 12 and 13 are recycled it is notadvantageous to provide a very high percentage of electrolyte depletion,or concentration, per pass. It is essential, however, to minimizeinternal leakage, especially of the concentrating stream into thediluting stream of either stack. This is because of the very high ratioof concentration which can be achieved only in a system with minimumleakage.

The frames and separators of the electrodialyzer stacks should be ofnon-conductive plastic construction appropriate to the operatingtemperature. The electrodes should be of a conductive materials, such asmetal or graphite, appropriate to the solutions. For cyanide coppersolutions the anode may be of stainless steel or an expendable anode ofcopper. The cathode may be of stainless steel or graphite. For acidchromate solutions the anode may be of lead and the cathode may be ofstainless steel. For chloride nickel plating solutions the anode may beof platinized titanium or expendable nickel. The cathode may be ofgraphite or of nickel. Selection of suitable materials of constructionis in all cases in accordance with principles and practice which arewell known.

In the use of the present invention with expendable anodes or wherethere is a metallic deposit on the cathode, the anodes and cathodes areremoved and replaced by methods well known to those skilled in the art.Optionally within the scope of this invention electrode wash streams maybe utilized to isolate the electrodes from the two principal streams.

Whereas the constructional features of the individual cells, includingthe membranes, frames and separators are, in accordance with thisinvention, the same as in the prior art the operation of the processdiffers in an important respect from that of the prior art. Specificallythe stacks are operated, in accordance with this invention, underconditions of concentration polarization at the interfaces of themembranes with the diluting stream.

1n the methods of the prior art the electrodialysis system is operatedto avoid the concentration polarization which is an essential element ofthe present invention in its preferred em bodiment. The reason foravoidance of concentration polarization in the prior art is to avoid theuse of excessive power but more important to avoid pH changes which leadto scale formation particularly on the anion selective membranes. Washsolutions used in the present invention are not subject to scaleformation in the operation of the electrodialysis stacks underpolarizing conditions.

It is well known that for a given electrodialysis system there is alimiting current density above which concentration polarization occursand that this limiting current is directly proportional to theelectrolyte concentration in the diluting stream. This limiting currentdensity divided by the electrolyte concentration is a linear function ofthe coefficient of mass transfer at the membrane-solution interface.This is, in turn, a function of a solution flow velocity and theconfiguration of the flow channel.

It is a purpose of the present invention to operate with a transferencenumber of the counterions in 'both anion-, and cation-, selectivemembranes very nearly unity. This is achieved by operation underconditions of concentration polarization of the membranes. Under suchconditions of polarization the ion concentration is nearly zero in thediluting stream solution at the membrane interface. Under such operatingconditions the concentration of co-ions in the membrane is exceedinglylow and the transference number of counter-ions is accordingly verynearly unity.

It has been discovered that if, in the system shown, the tanks 2 and 3are filled with nearly pure water at the start-up when power is appliedto the electrodes of stacks 7 and 8 and when the pumps 4, 5 and 6 arestarted to circulate the streams 11, 12 and 13, it is relatively easy tomaintain a condition of membrane polarization while the concentrationsin tanks 2 and 3 are increasing. As the operation of the dragout, 9 and10, brings electrolyte from the tank 1 to the wash tanks 2 and 3 thecurrent in each stack rises substantially in accordance with the faradayequivalence of the electrolyte in the dragout. This is maintained aslong as the voltage between each pair of electrodialyzer stackelectrodes is sufficiently high to maintain the condition of membranepolarization.

If the potential is less than that required to maintain the polarizingcurrent the membranes may pass into a condition of lower counter-iontransference number. This leads to a lower current efficiency andconsequently, a rise in concentration in the wash tanks, especially intank 2, as the dragout continues. The increase in concentration in thewash solution leads to further deterioration in membrane transferencenumber and the effect is cumulative leading to a much lowerconcentration ratio.

The higher the concentration in tank 1 and the greater the dragout inrelation to the effective membrane area the greater is the necessity formaintaining membrane performance as measured by the concentration ratio.This is true especially of complex cyanide solutions which have atendency to poison the anion-selective membranes by absorption resultingin a lower anion transference number and lower transport efficien cy.

As is well known a condition of membrane polarization in electrodialysisis indicated by a more or less sharp increase in the apparent resistancedefined as the rate of increase of voltage with current per cell pair.The increase in apparent re sistance is not always sharp because thesolution flow and electrolyte concentration are not always uniform overthe membrane surface. It has been discovered that in its application tothe present invention, membrane polarization is most effective inenhancement of membrane performance when the rate of increase of voltagewith current is greater than twice the resistance, i.e., when where I isthe current in the stack and V is the stack voltage. This condition maybe readily determined by measuring the voltages and currents in thesystem.

The application of this invention to the recovery of cyanide and coppervalues from a system for washing copper plated parts is illustrated bythe following examples.

EXAMPLES In a system illustrated the electrodialysis stacks 7 and 8consist of Aquachem Model WD 6-2 electrodialyzers made by Aqua-Chem,Inc., Waukesha, Wisconsin, with nine cationselective membranes, Ionac MC3470, and eight anion-selective membranes, Ionac MA 3475 each made byIonac, Div. of Ritter Pfaulder, Birmingham, NJ. The concentrating streamflowed through the cathode compartment and through alternate cells whilethe diluting stream flowed through the remaining cells and the anodecompartment. The efiective area of each membrane was 750 sq. cm and theflow rates were 225 gallons per hour for all streams.

The solution in tank 1 was initially made up with 8 oz/gall of coppercyanide, 9 oz/gall of sodium cyanide, 2.0 oz/gall of soda ash and 4.0oz/gall of caustic soda. With a voltage of 50 volts for stack 7 and 30volts for stack 8 the ratios of concentration in tanks 2 and 3 to thatin tank 1 were as follows for varying rates of dragout.

TABLE I Dragout Rate Liters per hour Concentration Ratios All IonsCompared Tank 2:Tank 1 Tank 32Tank l The operation of theelectrodialyzers at a voltage above membrane polarization is used toeffectively provide a ratio of electrolyte concentration of each stagein relation to a previous stage of at least 1:20 and preferably about1:100. The ratio of course may vary depending on the dragout rate andthe electrolyte concentration in the dragout. Ifthe dragout rateincreases sharply the concentration ratio may be only 1:5, but as thedragout rate falls off the concentration ratio will increase as the washstreams are continuously recycled through the dialyzer.

Although this invention is employed advantageously using two stages ofwashing and electrodialysis, it is not necessarily limited thereto. Thenumber of stages and the stack voltages and other parameters may bevaried without departing from the scope of this invention. Selection ofthe operating parameters is determined by the ratio of concentrationwhich is required and by considerations of power and equipment cost.

This invention may be applied advantageously to the recovery of thecomponents, including nickel chloride and nickel sulfate, of solutionsdragged out of nickel plating baths; of copper sulfate and fluoboratefrom copper plating baths. It may be applied also to the recovery ofcomplex fluorides of iron and chromium from the dragout of stainlesssteel pickle solutions and also to the recovery of gold, silver, andzinc values from cyanide bath dragout.

This invention has been described in terms of specific embodiments setforth in detail. Alternative embodiments will be apparent to thoseskilled in the art in view of this disclosure, and accordingly suchmodifications are to be contemplated within the spirit of the inventionas disclosed and claimed herein.

1 claim:

1. 1n the method of treating electrolyte from a solution on solids,wherein the solids are removed from a treating bath and passedsequentially through at least a first and second wash bath containing anaqueous solution,

the improvement which comprises,

pumping the solution from said treating bath through the concentrationcompartments of a first electrodialyzer and returning said solution tosaid treating bath,

pumping the aqueous solution from said first wash bath through thedilution compartments of said first dialyzer and through theconcentration compartments of a second dialyzer and returning saidaqueous solution to said first wash bath, and

pumping aqueous solution from said second wash bath through the dilutioncompartments of said second dialyzer and returning said aqueous solutionto said second wash bath.

2. The method according to claim 1 in which the electrolyte includes oneor more complex cyanides of copper, zinc, gold, silver or cadmium.

3. The method according to claim 1 in which the electrolyte includesalkaline chromate solutions.

4. The method according to claim 1 in which the electrolyte includesacid chromates and trivalent chromium.

5. The method according to claim 1 in which the electrolyte includesnickel sulfate or nickel chloride.

6. The method according to claim 1 in which the electrolyte containscopper sulfate or copper fiuoborate.

7. The method according to claim 1 in which the electrolyte containscomplex fluorides of iron, chromium, or aluminum.

8. The method of claim 1 wherein a series of said solids are passedthrough said wash baths, and the concentration ratio of electrolyte insaid treating bath with relation to said first wash bath is maintainedat least at 5:] and the concentration ratio of electrolyte in said firstwash bath in relation to said second wash bath is maintained at least at5: l.

9. A method of continuously recovering electrolyte from a metalfinishing bath dragout on solids which comprises sequentially immersingsaid solids into and removing them from each of a series of vesselscomprising a treating vessel and at least two wash vessels wherein eachof said vessels contain an aqueous solution more concentrated inelectrolyte than the aqueous solution in a subsequent vessel to whichsolids are transferred, continuously recycling a less concentratedsolution from a first one of said vessels between said vessel and thediluting stream cells of an electrodialyzer, continuously recycling amore concentrated solution from a second vessel through theconcentrating stream cells of said electrodialyzer, and operating saidelectrodialyzer to balance the electrolyte removed from the solids insaid second vessel by the transfer of electrolyte to said first vesselto maintain the ratio of electrolyte between said vessels at a ratio ofat least 5:1.

10. The method according to claim 9 in which the electrolyte includesone or more complex cyanides of copper,

zinc, gold, silver or cadmium.

1 1. In the method of treating electrolyte from a solution on solids,wherein the solids are removed from a treating bath and passedsequentially through at least a first and second 5 wash bath containingan aqueous solution,

the improvement which comprises,

pumping the solution from said treating bath through the concentrationcompartments of a first electrodialyzer,

pumping the aqueous solution from said first wash bath through thedilution compartments of said first dialyzer and through theconcentration compartments of a second dialyzer and returning saidaqueous solution to said first wash bath, and

pumping aqueous solution from said second wash bath through the dilutioncompartments of said second dialyzer and returning said aqueous solutionto said second wash bath.

12. In the method of treating electrolyte from a solution on solids,wherein the solids are removed from a treating bath and passedsequentially through at least a first and second wash bath containing anaqueous solution,

the improvement which comprises,

pumping the solution from said treating bath through the concentrationcompartments of a first electrodialyzer, pumping aqueous solution fromsaid first wash bath through the dilution compartments of said firstdialyzer and returning said aqueous solution to said first wash bath,

pumping an aqueous solution from said first wash bath through theconcentration compartments of a second dialyzer and returning saidaqueous solution to said first wash bath, and

pumping aqueous solution from said second wash bath through the dilutioncompartments of said second dialyzer and returning said aqueous solutionto said second wash bath.

13. In the method of treating electrolyte from a solution on solids,wherein the solids are passed sequentially through at least a first andsecond wash bath containing an aqueous solution,

the improvement which comprises,

pumping a solution through the concentration compartments of a firstelectrodialyzer, pumping the aqueous solution from said first wash baththrough the dilution compartments of said first dialyzer and through theconcentration compartments of a second dialyzer and returning saidaqueous solution to said first wash bath, and pumping aqueous solutionfrom said second wash bath through the dilution compartments of saidsecond dialyzer and returning said aqueous solution to said second washbath.

2. The method according to claim 1 in which the electrolyte includes oneor more complex cyanides of copper, zinc, gold, silver or cadmium. 3.The method according to claim 1 in which the electrolyte includesalkaline chromate solutions.
 4. The method according to claim 1 in whichthe electrolyte includes acid chromates and trivalent chromium.
 5. Themethod according to claim 1 in which the electrolyte includes nickelsulfate or nickel chloride.
 6. The method according to claim 1 in whichthe electrolyte contains copper sulfate or copper fluoborate.
 7. Themethod according to claim 1 in which the electrolyte contains complexfluorides of iron, chromium, or aluminum.
 8. The method of claim 1wherein a series of said solids are passed through said wash baths, andthe concentration ratio of electrolyte in said treating bath withrelation to said first wash bath is maintained at least at 5:1 and theconcentration ratio of electrolyte in said first wash bath in relationto said second wash bath is maintained at least at 5:1.
 9. A method ofcontinuously recovering electrolyte from a metal finishing bath dragouton solids which comprises sequentially immersing said solids into andremoving them from each of a series of vessels comprising a treatingvessel and at least two wash vessels wherein each of said vesselscontain an aqueous solution more concentrated in electrolyte than theaqueous solution in a subsequent vessel to which solids are transferred,continuously recycling a less concentrated solution from a first one ofsaid vEssels between said vessel and the diluting stream cells of anelectrodialyzer, continuously recycling a more concentrated solutionfrom a second vessel through the concentrating stream cells of saidelectrodialyzer, and operating said electrodialyzer to balance theelectrolyte removed from the solids in said second vessel by thetransfer of electrolyte to said first vessel to maintain the ratio ofelectrolyte between said vessels at a ratio of at least 5:1.
 10. Themethod according to claim 9 in which the electrolyte includes one ormore complex cyanides of copper, zinc, gold, silver or cadmium.
 11. Inthe method of treating electrolyte from a solution on solids, whereinthe solids are removed from a treating bath and passed sequentiallythrough at least a first and second wash bath containing an aqueoussolution, the improvement which comprises, pumping the solution fromsaid treating bath through the concentration compartments of a firstelectrodialyzer, pumping the aqueous solution from said first wash baththrough the dilution compartments of said first dialyzer and through theconcentration compartments of a second dialyzer and returning saidaqueous solution to said first wash bath, and pumping aqueous solutionfrom said second wash bath through the dilution compartments of saidsecond dialyzer and returning said aqueous solution to said second washbath.
 12. In the method of treating electrolyte from a solution onsolids, wherein the solids are removed from a treating bath and passedsequentially through at least a first and second wash bath containing anaqueous solution, the improvement which comprises, pumping the solutionfrom said treating bath through the concentration compartments of afirst electrodialyzer, pumping aqueous solution from said first washbath through the dilution compartments of said first dialyzer andreturning said aqueous solution to said first wash bath, pumping anaqueous solution from said first wash bath through the concentrationcompartments of a second dialyzer and returning said aqueous solution tosaid first wash bath, and pumping aqueous solution from said second washbath through the dilution compartments of said second dialyzer andreturning said aqueous solution to said second wash bath.
 13. In themethod of treating electrolyte from a solution on solids, wherein thesolids are passed sequentially through at least a first and second washbath containing an aqueous solution, the improvement which comprises,pumping a solution through the concentration compartments of a firstelectrodialyzer, pumping the aqueous solution from said first wash baththrough the dilution compartments of said first dialyzer and through theconcentration compartments of a second dialyzer and returning saidaqueous solution to said first wash bath, and pumping aqueous solutionfrom said second wash bath through the dilution compartments of saidsecond dialyzer and returning said aqueous solution to said second washbath.